Not applicable.
Not applicable.
ITU-T recommendation V.34 are standards for a modem operating at data signaling rates of up to 33,600 bits/s for use on the general switched telephone network and on leased point-to-point 2-wire telephone type circuits. The standard utilizes quadrature amplitude modulation for each channel with synchronous line transmission at selectable symbol rates including the mandatory rates of 2400, 3000, 3200 symbols/s and the optional rates of 2743, 2800 and 3429 symbols/s. The standard may use trellis coding for all data signaling rates. Trellis encoding is a method for improving noise immunity using a convolutional coder to select a sequence of subsets in a partitioned signal constellation. The trellis encoders used in the ITU recommendation are used in a feedback structure where the inputs to the trellis encoder are derived from the signal points.
Trellis coded modulation (TCM) is one of the coding standards recommended under the V.34 modem communications standard. Trellis codes using lattices of dimensions larger than two have been constructed and have several advantages. Two dimensional (2D) symbols are grouped in pairs to form four-dimensional (4D) symbol intervals. Multidimensional trellis code signals as a basis for signal constellations are a theoretical concept, since, in practice, multidimensional signals are transmitted as sequences of one or two dimensional signals. Doubling the constellation size reduces the minimum distance within the constellation, and this reduction has to be compensated for by the code before any coding gain can be achieved. Using a 4D signal set causes the constituent 2D constellations to be expanded by a factor of only square root of two, having half a bit of redundancy per 2D constellation.
The decoding operation comprises finding the correct path through the trellis that most closely represents the received binary sequence. The decoder finds a path for the received binary sequence that has the minimum Hamming distance from the received sequence. The iterative procedure accomplishing the decoding is the Viterbi algorithm. The algorithm uses forward dynamic programming to select the best, or minimum Hamming distance, path through a trellis. At each node, in the trellis, the only path retained is the best path, therefore limiting the number of retained paths at any time instant to the total number of trellis nodes at that time.
A TCM code is rotationally invariant if it contains the correct code sequence that gives transparent decoding to a phase rotation of the signal constellation. The decoder must be able to find a valid code sequence after rotation. The rotated TCM sequence must also map back to the same originally encoded bits of information as the unrotated sequence.
Representations of transmitted digital communications signal waveforms are expressed in a common analytical framework named the signal space. Signal space diagrams are graphs of digital signals as illustrated in two-dimensional vector format. Signal space diagrams are also called signal constellations. Signal waveforms can be sent with two-dimensional symbols, however a more efficient method is grouping 2D symbol intervals in pairs and sending them as 4D symbol intervals. Digital waveforms under the V.34 standard are sent using frames of digital symbols. A mapping frame consists of four 4D symbol intervals. A number of P mapping frames form a data frame. A data frame varies as 12, 14, 15, or 16 mapping frames depending on the symbol transmission rate from the encoder output. Depending on the symbol rate, seven or eight data frames (J) form a superframe. The duration of a superframe lasts approximately 280 ms. A superframe is the highest level entity in the V.34 framing definitions.
Bit inversions of the trellis encoder output are used for superframe synchronization purposes. An important function at a modem receiver is the synchronization process of the carrier frequency and phase synchronization as well as symbol timing synchronization on order for the receiver to operate. Synchronization is a relatively slow estimation process, and data detection a fast process, therefore usually those two operations are separated in real receiver implementations. The trellis encoder consists of a convolutional encoder which generates bit inversions for the purposes of superframe synchronization. Bit inversions are introduced in the 4D symbol interval in the beginning of each half data frame. The period of inversion is 16 when J=8 and 14 when J=7.
The bits at the beginning and middle of each data frame are inverted according to a periodic inversion pattern. The bit inversion pattern is a 90 degree rotation of the second 2D constellation point in the 4D symbol interval. The bit inversions continue throughout the encoding of the modulated signal stream. The inversions are performed by the transmitter, thereby causing the receiver to perform a reversal of the inversion pattern prior to decoding. The inversion pattern generator in the receiver and the reversal system in the receiver must be synchronized, otherwise a synchronization loss occurs.
The inversion patterns generally do not affect the distances in the path metric since the receiver should fully reverse the inversion prior to decoding. However, when a synchronization loss occurs, this loss introduces a measurable increase of the trellis metric. In most cases, this increase in the trellis metric is not noticeable by other error-detecting processes in the receiver, resulting in a reportedly good quality output from adaptive equalizer in the modem. However, the bits that are offset due to the synchronization loss from the expected symbol stream are decoded incorrectly. There is currently no method to detect the erroneously decoded bits derived from the synchronization loss since the receiver has no knowledge of content of the symbol stream that is being sent by the remote transmitter.
The present invention is a method to detect synchronization loss due to bit inversions in V.34 modem trellis decoding in a 4D transmitted symbol intervals due to a periodic inversion pattern that is used for superframe synchronization. Current technology provides no method to detect that the unsynchronized inverted bits are erroneous since the receiver can not map back to the originally unrotated bit sequence in the transmitter. The invention provides synchronization loss detection through comparing the moving averages of different sized block frames of data containing the inverted synchronization bits.
Modems sending packetized digital data under the V.34 format must maintain synchronization between the transmitter and receiver. To maintain synchronization, bits at the beginning and middle of each data frame are inverted by the transmitter. This method detects errors near frame locations containing the inverted bits by comparing moving averages of the minimum path metric for periodic user-defined groups of data blocks. The inversion slots in the data frames are periodic, appearing every first and sixth mapping frame.
For a data frame of twelve mapping frames, the detection method uses a mapping frame containing inverted synchronization bits as a first data block. The method then defines a second data block of six mapping frames. The first mapping frame of the second data block is the mapping frame of the first data block containing the inverted bits. The second data block also includes the next five mapping frames in series after the frame containing inverted synchronization bits. Synchronization loss is detected in a metric by creating a ratio of the average minimum path metric of the periodic series of mapping frames containing the inversion bits to the average of the periodic data block series of six mapping frames. The average of multiple frames in multiple periods having higher metrics will calculate to a higher average than a larger data block average, indicating a synchronization loss.